Conventional multilayer systems are produced by depositing materials with different refractive indices or different absorption coefficients on top of each other in several layers on a substrate. They are used in particular as mirrors in the extreme ultraviolet range. The wavelength range between 10 nm and 50 nm is designated as the extreme ultraviolet wavelength range. Other possible applications of multilayer systems are for example, in the visible wavelength range, as antireflective coatings of optical elements.
The reflection of electromagnetic radiation on a multilayer system is based on interference between the radiation which is reflected on the many interfaces of the multilayer system and is approximated by Bragg's law. This reflection is thus of a dispersive nature. The reflectivity of the interface between two such layers for electromagnetic radiation in a wavelength range<50 nm amounts to a few per thousand for angles that are greater than the critical angle. For reflection angles greater than the critical angle reflectivities up to a magnitude of 70% can be obtained. Multilayer systems are therefore used to achieve high reflectivities with maximum angles relative to the layer surface, and can also be used as dispersive elements.
A multilayer system for reflecting short wavelengths consists of successive sets of two or more layers respectively of materials with different refractive indices and thicknesses, for example in the magnitude of the wavelength of the reflected radiation. Partial reflection takes place at each of the interfaces between the different materials, and with a proper choice of the individual layer thicknesses, all partial reflections add up coherently. The overall reflectivity of a multilayer system is determined by the magnitude of the reflection per boundary surface, i.e. the difference of the refractive indices.
Multilayer systems for the extreme ultraviolet wavelength range generally consist of molybdenum-silicon- or molybdenum-beryllium-systems. For special applications multilayer systems are used made up from more than two differing types of layers. The choice of material with all multilayer systems depends heavily on the application's wavelength range.
Multilayer systems are utilized for the extreme ultraviolet to soft x-ray wavelength range amongst other things in lithography for the production of semiconductor components. It is precisely in their being employed in lithography that the multilayer system needs to demonstrate a long life with maximum possible constant reflectivity. On the one hand, the mirrors must show as little radiation damage as possible despite long periods of radiation. Any contamination or radiation damage would result in a shortened lifetime and usage interval, and hence in increased cost of the lithography process. The reflectivity may fluctuate, but would reduce on the long term.
Examinations have shown that when kept in air reflectivity decreases with time. Molybdenum silicon multilayer mirrors were examined. In particular, molybdenum used as the outermost layer became completely oxidized to molybdenum trioxide and molybdenum dioxide and contaminated with carbon-containing compounds, so that the absolute reflectivity decreased by 10 to 12%. The oxidization of the silicon layer into silicon dioxide caused a decrease in reflectivity of 4 to 5%.
In order to counter this it has been proposed that the mirrors should be provided with a protective layer of carbon of a thickness of 0.5 to 1 nm. With such mirrors we are dealing with a multilayer system for the soft x-ray range to the extreme ultraviolet wavelength range. For layering materials here use is made of, for example, ruthenium and silicon dioxide or even silicon carbide and hafnium.
In addition, to minimize the problem of reduction of reflectivity by oxidization and contamination of the uppermost layer, theoretical simulations were carried out for protective layers made from silicon dioxide, boron carbide, boron nitride, carbon, palladium, molybdenum carbide and molybdenum boride.
Moreover, it has been tested experimentally, how the reflectivity of multilayer systems changes when used in the context of lithography with extreme ultraviolet wavelengths. Measurements were carried out over a long period in real working conditions. In the course of this it was discovered that reflectivity is greatly decreased by contamination of the multilayer systems through residual substances in a vacuum.
A two or more layer passivation for multilayer reflectors for the soft x-ray and extreme ultraviolet wavelength range has already been described. The passivation consists at least of an under coating and an upper coating. In the case of the under coating it is a matter of the less absorbent material of the multilayer reflector, i.e. silicon in the case of molybdenum silicon mirrors, and beryllium in the case of molybdenum beryllium mirrors. In the case of the upper coating it is a matter of a material that does not oxidize or form an oxidation barrier and protects the layers beneath from oxidization. Quite generally these can be pure elements, carbides, oxides or nitrides. For example, silicon carbide, silicon dioxide or even molybdenum carbide are especially proposed. The thicknesses of the protective layers vary within the range of ca. 0.5 to 5 nm and are especially optimized on the mirrors to be protected. The upper coating is applied by precipitation from the gas phase or even controlled oxidization, the process for controlled oxidization not being elaborated in greater detail.
Another aspect is the optimization of the reflectivity. Theoretically a Mo/Si system can reflect up to 75% at near normal incidence at 13.5 nm. In practice, this reflectivity can not be achieved due to different imperfections in layer manufacturing. One of the severe limitations causing several percents drop in reflectance is imposed by formation of interlayers at Mo-on-Si and Si-on-Mo boundaries during the multilayer deposition. This reflectance drops further after subjecting the system to elevated temperatures, causing materials interdiffusion. The reflectivity drop is determined by the thickness of the interlayers formed, which initially (after deposition) ranges from around 0.8 to 1.5 nm depending on the deposition method. During annealing, the interlayer can only grow.
The forming of interlayers is a problem not only of Mo/Si multilayer systems, but of multilayer systems in general. One of the ways to prevent formation of these thick interlayers is to introduce additional layers between Mo and Si that can act as diffusion barriers.
A method for producing silicon nitride and nickel silicide layers in Ni/Si based multilayers has been described. Silicon nitride was formed by growing 3 nm Si, followed by the removal of 1 nm Si by 300 eV N+ ions. The next step was the deposition of a layer pair of 1 nm nickel and 2 nm silicon on the top. From the silicon layer 1 nm was removed by 300 eV Ne+ ions to produce a nickel silicide layer.
By investigating the influence of ion assisted deposition on the reflectivity of Ni/C multilayers, it has been found that the reflectivity was significantly enhanced compared with Ni/C multilayers deposited without ion beam assistance. An even stronger enhancement was obtained by ion etching every layer after deposition (without ion beam assistance) to smoothen the surface.